In the technical preparation of the dibenzothiazyl disulfides by oxidation of 2-mercaptobenzothiazoles, use has been made hereto of various oxidizing agents (Ullmanns Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A-26, pp 773-778, VCH, Weinheim, Basel, Cambridge, N.Y., Tokyo, 1995). For instance, the reaction with sodium chlorate and sodium-nitrite solution in hydrochloric medium at 30.degree. C. is the state of the art. This process, however, is burdened with a number of disadvantages. The consumption of mineral acid is very high (3 moles HCl per mole 2-mercaptobenzothiazole), and large quantities of by-products are formed. Furthermore it is known to carry out the oxidation of the 2-mercaptobenzothiazole by means of nitrous acid. According to the process disclosed in U.S. Pat. No. 1,908,935, 2-mercaptobenzothiazole is suspended in water, a water-soluble nitrite is added and the oxygen or an oxygen-containing gas such as air is passed through the reaction mixture. At the same time, a mineral acid which releases nitrous acid from the nitrite is added. In the case of the process according to U.S. Pat. No. 2,119,131 and U.S. Pat. No. 3,062,825, stoichiometric quantities of nitrite are employed as the sole oxidizing agent. By this means, a quicker and more complete reaction is achieved. These oxidation processes are, likewise, disadvantageous to the extent that, here too, the consumption of mineral acid is very high, and salts, as well as nitrogen oxides, accumulate in large quantities as by-products.
Chlorine has also already been employed as an oxidizing agent (Kirk-Othmer, Encyclopedia of Polymer Science and Technology (1970), Vol. 12, p 262). In this case, however, it is a matter of a complicated reaction with critical reaction conditions, in the course of which large quantities of over oxidized secondary products frequently accumulate. According to DE 23 09 584, for the purpose of increasing the product yield and diminishing the quantity of excess chlorine that is required for sufficient oxidation, separate currents of an aqueous solution consisting of an alkali-metal salt of mercaptobenzothiazole, an aqueous solution of an alkali-metal hydroxide and gaseous chlorine are caused to react with one another continuously at 20 to 75.degree., with vigorous stirring, beneath the surface of the liquid, whereby the pH value and the redox potential of the aqueous mixture are maintained by regulating the influx of the aqueous hydroxide solution and of the gaseous chlorine at pH 7 to 10 and at a redox potential of -150 to 250 mV. This process also requires very careful control in order to prevent further oxidation of the dibenzothiazyl disulfide to benzothiazyl-2-sulfinate and benzothiazyl-2-sulfonate. The process is also disadvantageous for the reason that large quantities of alkali hydroxide are consumed and large quantities of common salt are formed as a by-product.
Hydroperoxides such as hydrogen peroxide, alkali hydroperoxides and aralkyl hydroperoxides have likewise already been employed as oxidizing agents in the preparation of dibenzothiazyl disulfide (eg, DE 2,349,314).
The disadvantage, which is common to all the aforementioned oxidation processes is the requirement of comparatively expensive oxidizing agents and acids, bases or other auxiliary substances, and unusable by-products or secondary products also arise in some cases.
A process of the electrolytic oxidation of 2-mercaptobenzene to form dibenzothiazyl disulfide is also disclosed in DE 2,743,629.
There have also already been investigations as to whether the oxidation of 2-mercaptobenzothiazole to dibenzothiazyl disulfide can be carried out with oxygen as the sole oxidizing agent. With the process according to U.S. Pat. No. 3,654,297, this is possible if a cobalt phthalocylamine sulfate, disulfonate, trisulfonate or tetrasulfonate or mixtures of the same is employed as catalyst and the oxidation is carried out in an organic solvent that contains less than 15 wt. % water at temperatures from 50 to 80.degree. C. (cf. also Su 575 348; Chem. Abstr. 88 (1978), 89657 g). The production and industrial use of this catalyst, however, are problematic.
Finally, DE 2,355,897 shows the performing of the oxidation of the 2-mercaptobenzothiazoles to dibenzothiazyl disulfides by joint use of oxygen or an oxygen-containing gas and iron chloride, in particular iron(III) chloride, in a saturated aliphatic alcohol with 1 to 10 carbon atoms at temperatures between 0 and 150.degree. C. However, this catalyst results in a satisfactory rate of reaction only when it is used in a relatively large quantity, namely in a ratio of 0.8 to 1.5 mol per mol 2-mercapto-benzothiazole. The major disadvantage of this process, however, is that during the reaction, the iron precipitates out in the form of basic salts and the dibenzothiazyl disulfide that can be obtained is strongly contaminated with iron. A product obtained in this way cannot be used as a vulcanizing agent, for example, without elaborate purification.
According to DE 2,944,225 and DE 3,118,298, heavy-metal catalysts and amines are employed as a catalyst mixture. It is also possible for the amine to be replaced by ammonia.
The use of toxic heavy-metal catalysts such as Cd is problematic from ecological points of view; the residues of the toxic heavy-metal catalysts are, in addition, to be found in the final product or removed in an elaborate manner.